It is known that, to produce underground conduits or tunnels, even those of large dimensions, when the height of earth supporting the tunnel is limited, it is preferable to make a trench, to construct the tunnel in the open air and then to fill in the trench.
Depending on the cross section of the conduit, the latter can be constructed in different ways, for example of stonework or indeed of associated prefabricated elements. An original process has already been described, in applicant's European Patent No. 081402, for the construction of tubular hollow structures which may have a very large cross section, possibly corresponding to the size of the road, and which are intended to be positioned on the bottom of a trench which is filled in after such positioning. In this process, the conduit is formed by a plurality of associated longitudinal elements forming a group of adjacent segments placed end to end along a longitudinal direction, each element covering part of the cross section in the transverse direction and covering the length of the corresponding segment in the longitudinal direction.
To construct the conduit, a trench is thus opened down to the desired base level, the bottom of this trench forming a flattened resting surface on which there is positioned a certain number of segments placed end to end and connected to one another, for example, by two longitudinal girders embedded in the bed plate, and the trench is then closed.
In the embodiment described in the above-noted patent, each segment comprises a lower element forming a bed plate which rests on the bottom of the trench, two side elements positioned on either side of the bed plate and each equipped at its base with a stabilizing part allowing it to remain upright on the ground without being supported, and an arch element which rests on the upper ends of the side elements.
In another embodiment, European Patent No. 0181402, describes another type of structure which may also have a large cross section and in which each segment is formed by two elements, namely, a lower element forming a flat bed plate and an upper element in the form of a curved arch resting by means of its lower ends on bearing members made along the lateral sides of the bed plate.
In this manner, it is possible to produce conduits for the greatest variety of applications, for example sewage, water supply, underground passages, for example below a railway or a highway, for pedestrians, livestock, a river, etc.
However, it is sometimes necessary to construct structural systems formed by a plurality of associated conduits, allowing different uses. For example, an urban heating conduit may be associated with passages for electrical cables or various ducts, for water, gas, etc.
In rural locations, it may also be advantageous to associate with an underground conduit for the passage across a river another passage, for example for pedestrians or livestock.
When such a need becomes apparent at the time of construction, it is simple to produce in the same trench a plurality of conduits placed one next to the other, but it is then necessary to increase the width of the trench and thus the volume of earth moved and the areas compulsorily purchased.
Moreover, it is possible that the need to provide a supplementary conduit or one of larger passage cross section may become evident well after construction of the structural system.